Tunable broadband two-point-coupled ultra-high-Q visible and near-infrared photonic integrated resonators

Ultra-high-quality-factor ( Q ) resonators are a critical component for visible to near-infrared (NIR) applications, including quantum sensing and computation, atomic timekeeping and navigation, precision metrology, microwave photonics, and fiber optic sensing and communications. Implementing such resonators in an ultra-low-loss CMOS foundry compatible photonic integration platform can enable the transitioning of critical components from the lab- to the chip-scale, such as ultra-low-linewidth lasers, optical reference cavities, scanning spectroscopy, and precision filtering. The optimal operation of these resonators must preserve the ultra-low losses and simultaneously support the desired variations in coupling over a wide range of visible and NIR wavelengths as well as provide tolerance to fabrication imperfections. We report a significant advancement in high-performance integrated resonators based on a two-point-coupling design that achieves critical coupling simultaneously at multiple wavelengths across wide wavebands and tuning of the coupling condition at any wavelength, from under-, through critically, to over-coupled. We demonstrate critical coupling at 698 nm and 780 nm in one visible-wavelength resonator and critical coupling over a wavelength range from 1550 nm to 1630 nm in a 340-million intrinsic Q 10-meter-coil waveguide resonator. Using the 340-million intrinsic Q coil resonator, we demonstrate laser stabilization that achieves six orders of magnitude reduction in the semiconductor laser frequency noise. We also report that this design can be used as a characterization technique to measure the intrinsic waveguide losses from 1300 nm to 1650 nm, resolving hydrogen-related absorption peaks at 1380 nm and 1520 nm in the resonator, giving insight to further reduce waveguide loss. The CMOS foundry compatibility of this resonator design will provide a path towards scalable system-on-chip integration for high-performance precision experiments and applications, improving reliability, and reducing size and cost.